(a) Field of the Invention
This invention relates to slidable pistons which close the upper end of cartridge ampoules used in hypodermic syringes, the lower end thereof being closed by a pierceable diaphragm.
(b) Description of the Prior Art
In medical practice, hypodermic injections are sometimes administered subcutaneously, while others must be given intravenously, depending upon the particular medication to be administered. In either case, it is essential that the practitioner know with certainty, prior to injection of the medication, whether the hypodermic needle tip is located in a major blood vessel, such as a vein, or in subcutaneous tissue. Use of an aspirating syringe, in which a negative pressure can be generated in the syringe, affords a means of making such determination. Thus the appearance of blood in the syringe upon generation of the negative pressure would indicate location of the needle tip in a major blood vessel, while the lack of appearance of blood would indicate location of the tip in subcutaneous tissue. Depending upon the type of injection intended, the injection can then either proceed directly, or, if appropriate, the tip can be withdrawn and relocated.
Generally speaking, hypodermic syringes having aspirating capability belong to one of two classes, viz. manual aspirating and self-aspirating. In syringes of the former class, especially in those employing disposable, prefilled, medicament-containing ampoules, aspiration is effected by positively interengaging the syringe plunger rod with the slidable piston closing the upper end of the cartridge ampoule and slightly withdrawing the inter-engaged plunger/piston. There are many well known means of effecting such inter-engagement, the use of a barb or "harpoon" on the end of the plunger rod as disclosed by Mellott U.S. Pat. No. 3,224,445 being typical
Syringes of the self-aspirating class usually require rather elaborate and expensive physical mechanisms which can be caused to induce a negative pressure in the syringe barrel or within a cartridge ampoule when the syringe is of a type adapted for use with disposable ampoules. One means of inducing aspirating conditions in syringes of the latter type is the method disclosed by Ritsky U.S. Pat. No. 3,583,399, in which a fixed stud surrounding the inner end of a double-ended hypodermic needle inside the barrel of the syringe impinges upon the flexible diaphragm which closes and seals the lower end of the cartridge ampoule. Thus downward pressure upon the ampoule causes inward flexing of the diaphragm. Release of the downward pressure on the ampoule causes return of the flexible diaphragm to its original planar configuration with concomitant generation of sufficient negative pressure within the cartridge ampoule to initiate aspirating action.
Another means of generating aspirating conditions in cartridge ampoules comprises use of a slidable piston having a deformable section on the lower end thereof. Deformation of the deformable section is produced by means of a plunger rod which extends through an interior cavity within the piston. Thus in Cox U.S. Pat. No. 3,340,872, the desired deformation is effected by means of a thin extension on the end of the plunger rod.
The same result can be obtained by use of a plunger-within-a-plunger assembly as described by Evers et al. U.S. Pat. No. 3,295,525 and Brown U.S. Pat. No. 3,834,387, the inner plunger, in effect, replacing, and serving the same purpose as, the thin plunger extension described by Cox.
In each of the Cox, Evers et al. and Brown syringes, downward pressure with the plunger against the deformable section and release thereof produces a slight negative pressure sufficient to generate aspirating conditions. However, each of the pistons described by Cox, Evers et al. and Brown has the disadvantage that the piston must be molded with an interior cavity which adds substantially to its cost. This is particularly true of the Cox piston, which, because part of the cavity has an expanded portion therein, captures the mold thus requiring special molding techniques.